In a library system that a plurality of library apparatuses are only stacked, a storage medium is incapable of being transported between the library apparatuses. Therefore, while a drive unit of a first library apparatus of the library apparatuses is used, even if a second library apparatus of the library apparatuses has a drive unit which is not used, the drive unit which is not used is incapable of being used by the first library apparatus. Therefore, operation rate of the library system is low. Then, the library apparatus equipped with a medium transportation mechanism is developed. It enables the transportation of the storage medium between the library apparatuses. For instance, the medium transportation mechanism that provides a new elevator mechanism for transporting the storage medium between the stacked library apparatuses is well-known. A portion of space for cells and drive units arranged along movement area of a robot is used for the new elevator mechanism. U.S. Pat. No. 5,870,245 is an example of conventional technique.
A portion of space for the drive units and the cells in each of the library apparatuses is occupied for an area where an elevator moves. Therefore the number of the storage mediums which are mounted in the library apparatus equipped with the medium transportation mechanism decreases comparing with that of the library apparatus that is not equipped with the medium transportation mechanism.